Voltage controlled oscillators (VCO) are a key component of every integrated circuit, as most of today's electronics are synchronous systems, which require a system clock. A Local Oscillator (LO) in a radio frequency (RF) wireless system is also a type of VCO. Generally, the LO is the most critical block in the entire RF system as its performance determines many RF system parameters such as signal-to-noise ratio, image rejection, harmonic rejection, immunity to strong out of band blocking interferers, etc. Different types of VCOs that are typically used in industry include Ring Oscillators, LC Oscillators, Crystal Oscillators, Relaxation Oscillators, among others. Ring Oscillators employ several delay stages in a ring type fashion. Delay stages can be single ended or differential as shown in FIGS. 1A and 1B. When the number of stages exceeds two, the gain around the loop becomes bigger than 1 at 180 degrees phase shift and oscillation will start. If single ended delay stages are used, then odd-number of delay stages are needed. FIG. 1C shows a 3-stage single-ended ring oscillator and FIG. 1D shows a 5-stage single-ended ring oscillator. If differential delay stages are used, then even number of delay stages can be used to form the ring as shown in FIG. 1E. The delay of each stage is determined by the current in each delay stage and the output capacitance. Since one phase rotation travels through N identical delay stages (N representing a non-zero number), the phase difference between adjacent delay stages is 360/N and phase rotation is determined without ambiguity. This means that the phase at different delay stage outputs has a deterministic order, for example in FIG. 1B, the phase at Phi_0 is always leading Phi_1 by 60 degrees, the phase at Phi_1 is always leading Phi_2 by 60 degrees, etc. It would not be possible to have Phi_0 lagging Phi_1 by 60 degrees because that is a non-causal system and contradicts with the physics of the system.
For wireless applications, the LO normally uses a LC tank oscillator structure instead of ring oscillator due to its superior phase noise performance. FIGS. 2A through 2C illustrate possible implementations of the LC tank oscillator using a n-type metal-oxide-semiconductor (NMOS) transistor, a p-type metal-oxide-semiconductor (PMOS) transistor, as well as both NMOS and PMOS transistors as input and inductor Lp, Ln, and tunable capacitor Cvar as the load. An example of the LC tank impedance profile is shown in FIG. 2D. The oscillation occurs at the LC tank resonant frequency, which is:
  w  =            1              2        ⁢        π        ⁢                              L            ·            C                                .  
The quality factor of the tank ranges from 5 to 20 in today's silicon manufacturing process and this selective impedance profiling around the resonant frequency directly translates into a factor a 14 to 26 dB improvement in phase noise when one operates close to resonant frequency. This is the main reason that the LC tank oscillator dominates today's RF wireless applications.
In today's wireless applications, many RF system architectures employ low intermediate frequency architecture (Low IF). In Low IF RF systems, quadrature clock signals are required to perform image rejection. As shown in FIG. 3, a RF signal centered at frequency fRF undergoes frequency translation by mixing with both in phase (LO_I) and quadrature (LO_Q) clock signals from local oscillator which centered at fLO, after down conversion, the wanted RF signal is frequency translated to an intermediate frequency (IF) at fRF-fLO, but at the same time, the unwanted signal centered at 2fLO-fRF is also down converted to the same IF frequency. This unwanted signal at 2fLO-fRF is called the image component as it sits at the same frequency distance from the LO as the wanted RF signal. The image component can be much larger than the desired RF signal and will corrupt the mixer output to degrade system performance if not properly addressed. Thus, another block is needed after the mixer which is the poly phase filter. A poly phase filter is a network of resistors and capacitors. FIG. 4A shows a 1-stage poly phase filter, and FIG. 4B is a 2-stage poly phase filter. A poly phase filter provides a 90 degree phase shift between its outputs I and Q. After the poly phase filter, the signal component at fRF from both the I and Q mixer path adds up while the image component at 2fLO-fRF from the I and Q mixer paths cancel each other. Hence, the quadrature mixing and poly phase filter together provide a very important functionality, namely image rejection for low IF system.
Passive components such as resistors and capacitors match really well on chip (60 dB matching is easily obtained in today's fine line lithography), and the image rejection performance is mainly limited by the phase accuracy of the LO clock signals driving the mixer. Quick simulation shows that the phase imbalance between I/Q should be less than 0.2 degrees to have a better than 50 dB image rejection.
Traditionally, industry has used one voltage controlled oscillator to oscillate at twice the LO frequency and then divide down to obtain I and Q phase clock signals. This is shown in FIG. 5. However, as the LO clock frequency increases due to more and more bandwidth requirement from today's applications, the difficulty of designing the VCO, especially VCO oscillating at multi-gigahertz, becomes more and more challenging and poses a performance bottleneck for the overall system. A VCO oscillating at twice the desired LO frequency normally consumes much more power than twice the power of a VCO that oscillates at just the LO frequency and exhibits worse phase noise than 6 dB degradation compared to the VCO at just the LO frequency. Thus, it is of high interest to generate I/Q phase clock signals at the LO frequency instead of twice the LO. The conventional solution uses two identical LC VCO oscillating at LO and couples them together using active devices or passive components. FIG. 6A shows a VCO that generates I/Q signals through active device coupling. FIG. 6B shows a VCO that uses capacitive coupling for I/Q generation. For the second approach, the I/Q phase relationship is undetermined, which means I and Q will be 90 degrees different from each other, but Q can be either 90 degrees ahead of I or 90 degrees lagging I. An undetermined phase relationship causes down-conversion errors; e.g., unwanted image components will be down converted instead of the desired RF signal. Thus, active devices are still needed in the capacitive coupling approach to ensure no ambiguity in the I/Q phase relationship. However, using an active device to couple two VCOs degrades the I/Q quadrature phase accuracy. Due to random process variations, the final device that is manufactured on silicon varies in width, length, as well as device physical parameters such as mobility, threshold voltage, etc. Therefore, the coupling strength is different from device to device and obeys Gaussian distribution. This leads to the random variation of the I/Q phase relationship. Although bigger devices sizes improve matching and hence lower the random phase error, in multi gigahertz applications, a bigger device size introduces more parasitic capacitance, and hence an upper bound is mandated on device size. This translates to a lower bound of the variance of random phase errors. Also, to ensure no ambiguity of the I/Q phase relationship, the size of the active device needs to be bigger than a certain minimum threshold. This causes the I/Q clock signal to have a static phase error.
Accordingly, in today's wireless applications, LC oscillators are the preferred architecture of VCOs due to their phase noise advantage. In a low IF system, to generate quadrature clock phases for image rejection, either the VCO has to oscillate at twice the LO or the VCO has to oscillate at the LO to generate multiple phases. The conventional method of designing the VCO oscillating at the LO with active coupling has its limitations; to ensure a deterministic phase relationship between VCO's quadrature outputs, active coupling is needed. However, due to random variations of the device being manufactured, instead of an ideal 90 degrees phase angle difference, the I and Q clock signals suffer from a static dc phase offset as well as random phase errors. Both the static phase offset and variance of the phase errors have a lower bound and degrade system performance. Phase dc offset in the range of 1 to 2 degrees and standard deviation of random phase errors in the range of 0.5 degree is very common. Although the phase error between the I/Q clock signal is small compared to one cycle of oscillation, which is 360 degrees, 1 degree of phase error directly translates to about 30 dB of image rejection. To the overall RF system, this means that an external component is needed such as an expensive RF ceramic SAW filter to filter out strong image blockers. Therefore, there remains a need to develop a new architecture employing the benefits of ring oscillators and LC tank oscillators while also providing enhanced phase noise improvement.